Magnetron filter box

ABSTRACT

A magnetron filter box of novel construction serves to dissipate and filter out spurious signals appearing at the filament terminals of a magnetron. The filter box includes: two (or more) elongated electrical conductors spaced and insulated from one another which are provided with adjacent end portions of each adapted for connection to a respective one of the filament terminals of the magnetron which provide a current carrying path to the filament; microwave dissipative material of a predetermined volume is placed about and surrounds the electrical conductors minimally all along a portion of the lengths; and a metal container of predetermined dimension and volume encloses the microwave dissipative material and underlying conductors and is adapted to enclose additionally the stem or terminal end of the magnetron either with or without the metal pole piece as part of the container combination. The relative geometry between the dissipative material and the metal enclosure is such as to form a microwave cavity of a low-Q factor, suitably less than 100, and adjacent portions of the metal enclosure and the dissipative material is such as to define by analogy a transmission line with a &#39;&#39;&#39;&#39;lossy&#39;&#39;&#39;&#39; characteristic impedance, suitably less than 50 ohms.

atent Crapuchettes 21 Appl. No. 165,113

[52] US. Cl. ..31S/39.51, 333/69, 315/3953,

v 315/75, 331/86 [51] Int. Cl ..l-l0lj 25/50 [58] Field of Search..3l5/39.5l, 39.53,

[56] 1 References Cited 1 I V UNITED STATES PATENTS 3,531,613 9/1970Domenichini ..333/79 X 3,551,858 12/1970 Cielof... ..333/79 3,020,4472/1962 .Schall et al. 315/3951 X 3,543,082 11/1970 Boehm ..3l5/39.533,551,735 12/1970 Staats ...3l5/39.53- 3,458,753 7/1969 Staats..315/39.53

MAGNETRON FILTER BOX Inventor: Paul Wythe Crapuchettes, Woodside, Calif.

Assignee: Litton Systems, Inc., San Carlos,

Calif.

Filed: July22 1971 Primary Examiner--l-lerman Karl Saalbach Assistant.Examiner--Saxfield Chatmon, Jr. Attorney-Ronald M. Goldman et a1.

1 Apr. 10, 1973 [5 7 ABSTRACT A magnetron filter box of novelconstruction serves to dissipate and filter out spurious signalsappearing at the filament terminals of a magnetron. The filter boxincludes: two (or more) elongated electrical conduc tors spaced andinsulated from one another which are provided with adjacent end portionsof each adapted for connection to a respective one of the filamentterminals of the magnetron which provide a current carrying path to thefilament; microwave dissipative material of a predetermined volume isplaced about and surrounds the electrical conductors minimally all alonga portion of the lengths; and a metal container of predetermineddimension and volume encloses the microwave dissipative material andunderlying conductors and is adapted to enclose additionally the stem orterminal end of the magnetron either with or without the metal polepiece as part of the container combination. The relative geometrybetween the dissipative material and the metal enclosure is such as toform a microwave cavity of a lowQ factor, suitably less than 100, andadjacent portions of the metal enclosure and the dissipative material issuch as to define by analogy a transmission line with a lossycharacteristic impedance, suitably less than 50 ohms.

' l Clairns, 11 Drawing Figures PATH-HEB APR 1 01975 3; 727, 098

suwsur INVENTOR. PAUL w. CRAPUCHETTES EW "1 M ATTORNEY MAGNETRON FILTERBOX BACKGROUND OF THE INVENTION This invention relates to a magnetronfilter box and, more particularly, to a dissipative filter which isadapted to be used in conjunction with the filament leads of amagnetron.

Microwave vacuum tubes have heretofore been used as generators ofmicrowave frequency energy, microwave sources or, as variously termed,microwave oscillators, and as such are found in common use in manyelectronic systems. One such microwave tube is the magnetron. Themagnetron is, essentially, a selfcontained device in that the elementswhich cooperate to generate the microwave frequency energy are internalof the envelope. Terminals for connection of various electricalcircuitry, including filament terminals, which permit current to beapplied to the magnetron filaments, a cathode terminal, an anodeterminal, and an output terminal or window are provided outside the tubeenvelope. Very simplyexplained, when the appropriate sources of filamentcurrent are applied to the filament terminals and an appropriate sourceof high electrical voltage is applied between the anode and cathodeterminals, the microwave frequency energy is generated internally withinthe tube and this energy is delivered at the magnetron output window.

In magnetrons, as with all utilitarian devices, there is always somedegree of imperfection, however slight, which requires correction,compensation, minimization, or adjustment. In particular, the magnetronby design provides the predetermined microwave frequency at. thedesigned power level. However, there are in fact other frequenciesgenerated in varying degrees in varying levels, usually substantiallyless than the predominating output signal, such as harmonics of thefundamental frequency, and these signals may be termed spurious."Included in this definition of spurious signals are any portions. of thefundamental frequency which leak out through other than the outputwindow. These spurious signals in practice have either been so small asto be disregarded or were reduced to acceptable levels by conventionalelectrical filters, electrical devices which filter out or eliminate,ideally, the undesired spurious signals. In .recent years, however, themagnetron has found a prime commercial application as the source ofmicrowave energy used in microwave cooking appliances or ovens. And itis in connection with this appliance-that the magnetron and spurioussignals is next considered as background of my invention.

The microwave oven provides the means by which one can cook or heat foodby exposure to microwave radiation. In this manner food is heated orcooked very, very rapidly in comparison to cookingwith conventionalovens and such a decrease in cooking time is an advantage and representsa significant convenience for many people.

As with all technicallysophisticated appliances, however, special careis given to the design and maintenance of the appliance in order tomaintain its utilitarian values while avoiding abuse. While microwaveradiation is of utility in cooking food, it is not to be exposed tohumans or interfere with other electronic equipment in any significantdegree.

walls prevent radiation leakage from these areas. A

door closes the cooking chamber during operation of the magnetron.

Any leakage of radiation, either from around the edges of the doorclosing the cooking cavity or from the back panel of the oven housing inwhich the magnetron and associated power supplies are physicallyenclosed,

is to be avoided.

A first consideration must be made regarding leakage of microwaveenergy. The Federal'Communications Commission, another Governmentagency, prescribes and allocates the frequencies which may be used forvarious purposes and has assigned the frequency of 2,450 MHz formicrowave cooking. By regulation, the FCC requires the radiating unit tobe of such a construction that intensity of radiation leakage atfrequencies outside of a percent bandwidth of the assigned frequencyrange of 2,400 to 2,500 MHz must be less than 25 microvolts per metermeasured at a distance 'of1000 feet per 500 watts of output. The purposeof this limit is to prevent signals from the microwave oven frominterfering with other forms of radio communications and television.

A second consideration regarding the degree of radiation is made.Initially, such radiation leakage as was permissible and harmless tohumans was originally determined to be on the order of 10 milliwatts persquare centimeter at a distance of 2 inches. This represents a level ofradiation comparable to that normally emanating from the sun on a clearspring day. When one considers that the magnetron delivers into the ovencavity some 500 to 1,000 wattsof microwave energy, such leakagerepresents a rather small portion. Oven designs and equipment thenavailable met this standard and were thus considered safe. However,recently the heretofore permissive radiation limits were re-examined,particularly having in mind the increasing number of electronicappliances being introduced into the home by the consumer and to thepossibly cumulative effects of harmless doses of radiation from each.Thus, to provide additional protection the levels of permissibleradiation were reduced even further as an attempt to account in some wayfor the unknown amountsof radiation that the consumer may be exposedfrom an unknown quantity of additional appliances, each of which bythemselves are harmless. Accordingly, by Government regulation, thepermissible radiation leakage whether from the door seal area attentionto manufacturing tolerances and more sturdy and expensive sheet metalwork.

By comparing regulations of the two agencies one determines essentiallythat of the two, the restrictions on spurious out-of-band radiationpromulgated by the FCC to prevent interference with radio and televisionreception stations is more restrictive by some 60 db than the permissiveradiation levels announced by the other Government agency. However, thepermissible limits of radiation of the latter are more strict in adifferent sense, they include leakage at the assigned frequency of 2,450MHZ or in-band radiation.

One type of magnetron widely used in microwave ovens is the L-500l orvariations thereof,v manufactured by Litton Industries, Electron TubeDivision, San Carlos, Cal., which are perhaps better illustrated in US.Pat. Nos. D-215,298, D-208,86l,' D- 2l8,l69, and D-2l2,758. I

The filament terminals on the magnetron, as previously noted, physicallyappear as bands of conductive material on a stemlike projection at oneend of the magnetron. The stem is actually divided into'electricallyinsulated parts separated by aluminum oxide, an insulating material,with the tip of the stem comprising a pinched-off copper tube. This stemend of the tube was inserted within or surrounded by a large metal orlarge shielding enclosure referredto as the choke box or magnetronfilter box, and this latter unit'was added during assembly of themagnetron into the equipment. Such an element appears in two of thecited patents. Typically, one end of this enclosure was formed of ametal screen material or a perforated metal sheet so that air could beintroduced into that region. The openings in the enclosure are, however,of such dimension as to prevent the passage of any spurious signalsabove a predetermined intensity from the vicinityof the cathode stem.The power supply leads including the high voltage cathode supply and thetwo leads through which filament current was coupled to thefilamentter-' such sources considered, the normal level of spurious radiation inpresent ovens is below the FCC limits.

Conservative design principles require the manufacturer to provide anoven with components which have leakage, if at all, substantially belowthe 25 microvolt per meter level prescribed by the FCC so thatconsidering the vagaries of normal mass production manufacturingtechniques, all ovens built should comply with the radiation standards,regardless of the degree of per} fection of assembly by personnel on anygiven day and in this way waste or scrap in the manufacturing processis'eliminated.

and filament, which also serves as the cathode, inuring due toasymmetries in the anode. From the filament the microwave energypassesthrough the terminal 'on the stem and appears on the power supply leads,unlessthere filtered out or dissipated.

A second source of spurious signals are subharmoriics. These aremicrowave-signals of some submultiple 'of the fundamental frequency of2,450 MHz which are inherently generated in the tube but in very, verylow' levels. A prime objection to such subharrnonic signals is that theymay fall within the television frequencies and thus could potentiallyinterfere with normal television reception during operation of themagnetron.

minals of the magnetron extendedfrom the outside of the enclosurethrough an opening to the magnetron stem. r

The filter box typically included wire inductors and capacitors. Theinductors were placed in series with the respective filament leads andthe capacitors were placed between a filament lead and electrical groundto form a low pass filter. Such a filter prevented beyond permissiblelimits any high frequency signals which might leak from the cathode tothe filament terminals from traveling along the electrical leads to theoutside of the oven enclosure, and the capacitors served to shortcircuitto ground as much high frequency energy as was possible. In addition, oras an alternative construction, beads of lossy" or dissipative microwavematerial such as ferrites were included in such a filter box topositively absorb and dissipate any microwave energy in that vicinity.While such constructions proved acceptable in the past, costs must bereduced and the recent reduction in level of permissible radiationleakage requires reevaluation of design and manufacturing procedures.

A third form of spurious signal derives from the multiplicity ofresonant modes of the multi-segment anode found withinthe magnetrontube. If the anode has N resonators there will be N resonant-modes ofthe first order, second order, etc. Since the normal mode of operation,called 1' mode, has been optimized for impedance all other signals arecomparatively lower. Since these other resonant modes will always beoutside of the allotted frequency range band allotted fro microwavecooking and heating, care must be exer- I cised to assure that energyproduced and radiated is less than the permissible level.

A fourth form of spurious signal derives from an electronic interactionbetween the impedances associated with the normal 41" mode andunintended systematic impedances. Generally such signals, if generated,are within 2,415 and 2,485 MHz where their magnitude can exceed 25uv/meter, as described above.

OBJECTS OF Tl-lElNVENTION microwave frequency signals that may appearatthe filament terminals of a magnetron.

And it is a still further object of the invention to provide magnetronswith integral filters for use in microwave ovens that reduce further anypotential leakage from the microwave oven.

BRIEF SUMMARY OF THE INVENTION In accordance with my invention, at leasttwo elongated electrical conductors are spaced from one another and haveadjacent end portions adapted for connection to filament terminals of amagnetron and form a portion of the electrical path by which filamentcurrent is supplied to such magnetron. Microwave dissipative material islocated along the length and substantially surrounds the two conductors.In addition, a microwave cavity comprising metal walls surrounds andcontains within the loss material and underlying conductor portion, theconductor ends and the magnetron filament terminals. In accordance withthe invention an opening is provided in this metal cavity into which amagnetron stern portion containing the filament and high voltageterminals can bev inserted for connection to the respective end portionsof said electrical conductors and a passage is provided in a remote endportion of the cavityto permit suitable connections to be made with saidconductors.

Further, in accordance with the invention, the microwave cavity is of alow Q factor, suitably less than 100. In accordance with said low Qfactor, the ratio of the volume defined by the microwave cavity to thevolume of the loss material is a number of fraction greater than I andlesser, approximately, than 20.

In accordance with an additional aspect of the invention the outercavity walls and the elongated conductive wires and attached lossmaterial are considered analogous to a coaxial transmission line and thegeometry of such line is defined to have a characteristic impedance Z,preferably less than 50 ohms.

In accordance with a still further aspect of the invention one wall ofthe microwave cavity may comprise the flat pole piece section of themagnetron.

The foregoing and other objects and advantages of the invention togetherwith the elements comprising the invention, including equivalents andsubstitutions for the elements thereof, become apparent to one ofordinary skill in the art from a consideration of the following detaileddescription taken together with the figures of the drawings in which:

DESCRIPTION OF DRAWINGS of the elements in the preferred embodiment andtheir method of assembly; and

FIG. 6 illustrates a top view of another embodiment of the inventionwith the upper container wall removed; FIG. 7 and 8 illustrate side andend views of the embodiment of FIG. 6;

FIG. 9 illustrates still another embodiment of the invention;

FIG. 10a represents in cross section the conductors and loss material ofFIG. 9; and

FIG. 10b represents a modified form of the conductors and loss materialsuitable for the embodiment of FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 illustrates, symbolically,the elements of my invention in a magnetron filter box and itsrelationship to a magnetron, symbolically illustrated, used in microwaveoven. The magnetron is of the type L- 5260A sold by Litton Industries,Electron Tube Division, illustrated in greater detail in U.S. Pat. No.D- 212,758 and U.S. Pat. No. 3,493,810. The magnetron includes a bodyportion I, typically a hollow copper cylinder, containing therewithin ina vacuum the elements of the magnetron that are necessary for thegeneration of microwave energy including cathode, anode, filament, andmagnetic pole piece extensions, which details are not illustrated orotherwise explained since they form no part of the instant invention andare available elsewhere in the prior art. At one end of the magnetronbody is an output terminal of window 3 which is the exit for themicrowave energy generated within body 1. At the other end is a stemportion 5. Stem 5 extends from the body 1 and includes spaced metalportions or terminals 7 and 9 which internally complete a circuit to themagnetron filament winding. The terminals are insulated from one anotherby a vacuum tight aluminum oxide insulating material or spacer. Bothstem 5 and window 3 protrude through openings provided in pole pieces 13and 15. The pole pieces are of a flat rectangular shape and consist offerromagnetic material, typically annealed iron, and function as part ofthe magnetic circuit necessary to the operation of the magnetron. Afirst permanent magnet 17 is located between and at one end of polepieces 13 and 15, and a like second permanent magnet 19 is located atthe left end and between the same pole pieces. Both magnet 17 and magnet19 have their North Poles facing pole piece 13. The magnets provide themagnetic flux which is carried through the magnetic path formed by thepole pieces and by means of magnetic material connected at the front andback ends of magnetron body 1', and internal pole pieces (notillustrated) present the magnetic flux internally of the tube where suchforms a necessary ingredient to the generation of microwave energy. Thehorizontal lines 21 are symbolic of the metal, suitably aluminum,cooling fins which are stacked in between the pole pieces in a heatconducting relationship with the body 1. The cooling fins containsuitable openings to permit them'to be fitted over the magnetron body 1and magnets 17 and 19. Obviously in the variation of the magnetronillustrated in U.S. Pat. No. D-2l8,l69 the cooling fins would bearranged vertically and radially outward from body portion 1.

A pair of elongated electrical conductors or wires 31 and 33 areillustrated spaced from one another above pole piece 13. Electricalconductors 31 and 33 include respective end portions 35 and 37 locatedproximate one another and connected, respectively, to filament terminals7 and 9. These conductors extend through passages in and support threebodies of a dissipative or loss material 39. The passages, not Clearlyillustrated in 1 this figure, permit the loss material to be mounted sothat it surrounds all, portions of each conductor and at the same timeserves to maintain the predetermined spacing between conductors 31 and33.

Dashed line- 41 represents the walls of a compact metal container whichencloses the ferrite material and a portion of the wires 31 and 33including the end portions 35 and 37 thereof. Thecontainer covers, also,and

encloses the stem end of the magnetron. In this illustration one wall ofthe metal container 41 is effectively formed by the flat surface of polepiece 13. The relationship between the metal, can or cover and thedissipative material will be discussed hereinafter in greater detail.Loss material in addition to 39 is included in the metal cover. Anadditional slab of microwave loss material 40 is attached to an end wallof the container.

An insulator 43 is inserted in a passage in an end wall of thecontainer. The insulator contains two passages spaced from one anotherthrough which wires 31 and 33 extend, and the insulator serves toprevent the wires from contacting the metal walls of container 41. It isapparent that the particular conductors 31 and 33 need not be made solong as to extend beyond the confines of container 41 but instead may he-terminated at a socket fixed at the end wall. However in such aninstance the extension of electrical circuitry would requiretheinclusion of a plug fitted to be received with such a socket so as topermit anextension of the electricalpath through the containers beyondthe walls of container 41 to the appropriate source of filament current.The foregoing description provides background to the more exactdescription of the preferred embodiment'. As can be seen the magnetronfilter box is formed in a small and convenient package. suitably mountedintegral with the magnetron. In connection with the remaining figures,it is noted that when an element is assigned a numeral in a precedingfigure that same numeral is thereafter used, for convenience of thereader, in subsequent figures where the same element appears.

The top view of the preferred embodiment of the magnetron filter box isillustrated in FIG. 2 with a top wall or sideremoved inorder to providebetter illustration of the contentsrA container or box which forms amicrowave cavity is of an elongated narrow construction and is formed ofmetal, suitably aluminum or stainless steel. This includes a pair of endwalls 50 and 51, a front wall 53,'a back wall 54, a bottom wall 57 and atop wall (removed 55 in FIG. 3). End wall 51 includes a v passage inwhich is mounted an insulator, 43 in FIG. 1.

dashed lines, is mounted spaced from and generally parallel with thefirst conductor 31. Conductor 33 includes a front end portion 37proximate end portion 35 of conductor 31. Suitably, conductors 31 and 33used are Teflon insulated, as made by Gore Industries, to eliminatecorona. End portions'35 and 37 are bent so as to engage or contactsatisfactorily the filament ter- I minals of the magnetron stem,represented by the dashed lines 9 and 7. In this embodiment conductors31 and 33 are of such a length as to extend through wall 51 and theinsulator 43, mounted in wall 51, so that they are maintained spaced atthatend and insulated from electrical contact with the metal containerwalls. Beads 39a, 39b and 390, of microwave dissipative or lossmaterial, suitably ferrite, are mounted on conductors 31 and 33 alongthe straight elongated portion thereof within the metal container. Eachof the ferrite beads is a standard material,- usually grade 9 or 11, andhas an elliptical cylindrical geometry and the consistency of a solidsintered pressed powder body. Each of the beads has two spaced passagestherethrough through which conductors 31 and 33 extend. In this way theconductors 31 and 33 support the ferrite beads and, in turn the ferritebeads assist to maintain the conductors spaced apart. In addition, it isseen that the ferrite A sheath 4 2 of heat shrinkable tubing, suitablyirradiated polyvinyl tubing, such as that sold by Raychem Company, whichmay include additional loss material, covers the beads 39a, 39b and 39c.To permit a clear illustration of the beads the cover tubing isrepresented solely by dashed lines. i

In addition, a standard size slab of ferrite microwave loss material 38is attached to end wall 50, suitably with epoxy.

Bottom wall 57, represented by a dashed line in FIG. 3, of the filterbox. in the preferred embodiment is in fact a portion-of the flat ironpole piece of the magnetron, such as illustrated in FIG. 1. An opening59 represents the opening in the pole piece through which the magnetronstern containing'the filament terminals and cathode terminals extends.The side walls 50, 51, 53 and 54, are simply joined to the metal polepiece in a manner which eliminates any gaps or vcracks therebetween.This is suitably accomplished by welding or by electrically conductiveepoxy material. The magnetron pole piece thus serves to provide anelectrically conductive body or wall to the metal container with anopening adapted to receive the end of the magnetron. However, it isapparent that in an obvious variation of this embodiment of theinvention, a'metal wall can be provided to close the bottom of thecontainer before the box is placed atop the magnetron pole piece. Ofcourse, a suitable opening or passage such as 59 would be formed at thelocation in the figure to permit the insertion therethrough of themagnetronstem. In such a construction the bottom wall is fastened to themagnetron pole piece and the pole piece serves only to support thefilter and, eliminates its function as an ele.ctrically conductiveboundary. I

FIG. 3 illustrates the filter boxfrom one side facing wall 53 but withside wall 53 and attached ferrite removed, the embodiment of FIG. 2. Endwall 50, insulator 43, installed in the passage through side wall 51,electrical conductors 31 and 33, in part illustrated'by dashed lines,and the respective front end portions 35 and 37 thereof, are presented.In this figure the conductors are also shown extending through theferrite beads 39a, 39b and 390, and the ferrite beads are enclosed intubing 42. In addition, ferrite slab 38 is shown attached to the endwall 50, and air openings 56 are visible in back wall 54. Bottom wall 57in this view is represented by dashed lines since in the preferredembodiment the electrically conductive wall is formed by a portion ofthe surface of the pole piece of the magnetron and in the alternativeconstruction, discussed in connection with FIG. 2, may be a separatemetal wall. The stem portion of the magnetron is represented by thedashed lines and includes a pinch off tube and filament terminals 7 and9 shown extendingthrough the opening59 in this bottom wall. The top wall55 of the container includes a raised portion, bulge, or dome 60 at theend of the container adjacent end wall 50 and over the opening 59. Thisraised portion forms a dome over the end of the stem of the magnetronand serves to maintain a proper insulating space between the metal partsand the end of the magnetron stem while otherwise permitting a smallerdimension or height to the container elsewhere than at this location,thus reducing cavity Q.

The dome portion 60 of top wall 55 includes several holes or passagestherethrough, such as 61a, 61b and 610, visible in this figure. Theseopenings are generally less than 1/ 10th of the wavelength of thefundamental microwave frequency of 2,450 MHz so as to preclude anyleakage of microwave frequency therethrough and at the same timepermitting a passage for air to cool the magnetron stem. I

In the structure of the preferred embodiment the top wall 55 and bottomwall 57 are enlarged at the right hand ends in FIGS. 2 and 3, primarilyto make the container large enough in volume at that end to receive thestem of the magnetron and allow sufficient clearance therebetween so asto provide appropriate high voltage insulation and prevent corona orhigh voltage breakdown therebetween. At the left hand end of these wallsthe width of the wall is significantly more narrow so as to provide aslender appearance, preferably with a slight spacing between the endwalls and the ferrite beads. Overall the top view of the top and bottomwalls is seen to resemble in shape paddles with handles. As a result,the front and back walls, 53 and 54, are bent in shape so as to conformto edge of the top and bottom walls. End front wall 50 is rectangular inshape but of a larger area than the other end wall 51.

The end view of 'FIG. 4 illustrates the filter box with end wall 51 andinsulator 43 removed. Viewing the filter box from that end, electricalconductors 31 and 33 are visible and extend through passages in ferritebead 39a. For clarity, sheath tubing 42 and ferrite slab 38 illustratedin FIGS. 2 and 3 is omitted. Top wall 55 is seen to include the domeportion 60 and in this view an additional hole, 61d, is visible, inaddition to the holes 61a and 61b visible from theview of FIG. 3.

The metal walls of the container form essentially a closed microwavecavity in which any microwave energy is confined. By design, themicrowave cavity in the filter box of the invention is of a low Q, 0representing the quality factor of the cavity, and thus is considerablysmaller in size than the shielding cavities of the prior artconstructions. The ferrite material installed within the cavity occupiesa predetermined volume, represented by V,. In turn, the volume of thecavity is of a predetermined volume, represented as V,,. In order toobtain a low Q cavity on the order of one-hundred or less, therelationship or ratio between V and V, or V /V, is such as to be anumber, including any fraction thereof, approximately less than 20 andatleast slightly greater than the number 1 (one).

A second factor used to determine the relative geometry and actualdimension of the metal container and ferrite of the preferredembodiment, FIGS. 2 through 4, is best illustrated in connection withFIG. 4. Considering the narrow length of the metal container (apart fromthat over the stem of the magnetron) as analogous to a transmission linein which the electrical conductors 31 and 33 and the surrounding ferriteare considered as the center conductor thereof, an approximatecharacteristic impedance 2,, can be assigned. In the preferredembodiment of the invention this characteristic impedance is as low asfeasible, preferably less than 50 ohms. An approximation in obtainingsuch impedance is that there is a mean perimeter such as represented bythe dashed lines P in FIG. 4. This means perimeter is the averagebetween the perimeter of the ferrite material and the perimeter of theinner surface of the metal container. In addition, a mean or averagespacing s between the ferrite material and inner surface of the walls ofthe metal container is defined, disregarding the end ferrite slab 38which is not a part of the cross-section. Accordingly, the impedancecharacteristic Z is approximated as 377 (S P). It is noted that in theinstance where the metal container is formed such as to directly contacttheenclosed ferrite material, then S reduces to approximately 0 and theperimeters of the ferrite, the container, and the mean perimeter Papproach a common value for which there is no limiting impedance 2,. Theaddition of ferrite such as 38 at the end wall 50 reduces further 0,, orquality factor of the entire enclosed cavity.

In an actual construction of this embodiment of the invention thefollowing dimensions for the container were chosen and are presentedhere by way of an example. At its widest end top and bottom walls 55 and57 of the filter box are 1% inches wide; at their narrow end these wallsare /8 inch wide. In addition, the transition between wide to narrowwidth occurs at a location 1% inches from either end, and the length ofthe top and side walls is 3 inch overall. The height of the front andback walls is of an inch.

By contrast to the invention, one prior art construction had a filterbox 1% inches deep by 4 inches in width and 5 inches in length. Suchprior art filter box contained four 1 inch balun coils and two filtercapacitors. In addition to requiring many difficult assembly operations,the costs were high and the results were barely equivalent to thatobtained in the filter box of the invention. In another prior artconstruction the filter box was 1% inches deep by 4 inches in length and3 inches in width, and incorporated therewithin inductive components.Not only is this of a larger size and of a relatively more expensivecost of manufacture, but the results were poorer by some 20 dbs, whichincreased the possibility of television interference and required thepower supply enclosure to be very tight.

I construction is given. Head 39 cross-section is /16" X 9/16" so thatits perimeter, P,, is [5/16 1r 5/16) I X 2] which reduces to 1,605inches The height of the box is inches and its width is "/a of. an inchso its perimeter, P,,, is "/s "/a] or'3.25 inches. The mean perimeter,P, is approximately equal to the sum of P, and P b (1.605 3.250) =32(4.855)

2.428 inches. To determine, S, the mean spacing as heretofore defined,the outer diameter of the ferrite bead 39 is the effective workingsurface, since, the dielectric constant, for the bead is 13.5. Thus Sinch-the height of the boxless the height of the bead, 5/16 multipliedby k 0.219 inches. Thus, Z, 377 X 0.219/2.428 34.0 ohms. Note that ifin' determining, S,-the width of the box and the widthof the ferritebead were used to determine the average spacing a value of Z as defined,is obtained of 24ohms.

To determine the quality factor Q in this practical construction asherein defined and taught, the volume, V of the enclosed ferritematerial is first determined and equals the sum of volumes of the threeferrites' 39a,

In operation a suitable source of filament current is supplied toelectrical leads 3] and 33 and the latter conductors carry this currentto the filament terminals of the magnetron. Should any microwavefrequency energy be coupled from the cathode terminal into the stem areait is ideally absorbed by the ferrite material 39a through 39c and 38.The ferrite material thus serves to eliminate any high frequency energywhich couples from the cathode to the filament leads 31 and ,33.In'addition, the entire combination of metal cavity and ferrite materialconstructed according to the preceding teachings present a load to boththe fundamentaL'to any. harmonics of the fundamental, and to anysubharmonics of the fundamental energy. This essentially loads down thetube and inasmuch as the characteristic impedance Z, of the unitconsidered as a transmission line is low,'.suitably below 50 ohms, any

' possibility of propagating beyond the filter box any so- 0.470 cubicinches. The volume of the container, V is according to the exemplarydimensions previously I V,,V, z 2.657/O.470 5.6

The measured attenuation for this unit was 60 dbm and the measuredattenuation for the FCC limit on the same scale was 54 dbm.

The exploded view of the filter box illustrated in FIG. Sillustrates onemethod of assembly of the filter box of FIGS. 2-4. The insulation coatedelectrical conductors 31 and 33 are relatively rigid and their endportions 35 and 37 may be first formed to the desired shape. The ferritebeads 39a, 39b, and 39s, are thereupon slipped over the insulatedconductors. Additionally, the ferrite slab 38 is cemented in place tothe end wall of the can or metal container. The heat shrinkable tubingis placed over the ferrite beads and shrunk in place to I form a goodfit. Thereupon the ends'of the wires 35 and 37 are attached to the endsof the magnetron, suitably by soldering, to form a firm electrical pathand mechanical joint therewith. Next the can is moved down and the wires31 and 33 slipped into the passage in the can and the can is then placedover the entire assemblyand in abutment with the pole piece, notillustrated, of the magnetron. Subsequently, insulator 43 is slippedover the ends of electrical leads 3] and 33 and pushed into positionwithin the opening in the can. Subsequently the can is welded to thepole piece of the magnetron to form a complete unit.

dissipated inthe air atmosphere surrounding the metal called fast modesis eliminated.

Another embodiment of the invention is illustrated in FIGS. 6, 7 and 8showing the magnetron filter box from various views. For convenience,elements in FIGS. 6, 7, and'8, which find a corresponding element in thepreceding'embodiment of the invention of FIGS. 2 through 5, aresimilarly identified with a corresponding numeral which is primed.Moreover, since each of those'elements has been discussed in connectionwith the preceding preferred embodiment of the invention and have thesame'function and purpose, it would appear-redundant to repeat in detailthat description which is already in the readers understanding. Fordescription of those elements reference should be made to thepreceding'description. For conciseness, therefore, the description ofstructure following is, in

I essence, a description of features different from that found in thepreferred embodiment with some comparison between the two. 3 I

In this embodiment it is first noted that the container walls areessentially rectangular in shape. Accordingly, that portion of the metalcontainer surrounding the ferrite beads, 39a through 39c, is spacedfromthose beads by a slightly larger distance and in this forms a basictransmission line, analogously, of a lossy characteristic impedance Z,slightly larger than that of the embodiment of FIGS. 2-4. In addition,slabs of ferrite material, 40a, 40b, 40c and 40d, are installed withinthe metal container and provide additional loss material. Ferrite slabs40a and 40b are cemented or epoxied to front walls 53', and ferriteslabs 40c and 40d are cemented or epoxied to back wall 54'. While it ispossible to obtain the same volume of ferrite within the container byvsimply increasing the dimensions'of ferrite beads 39a, 39b and 39c, todo so would result in a design that requires ferrite beads ofanonstandard size and hence of a substantially more expensive per unitvolume cost. It is noted that the size and geometry of ferrite slabs 40athrough 40d are such as is commercially available as standard items. Anadditional advantage of locating these standard ferrite elements 39'along the walls of the metal container accrues: any heat built up in theloss material through dissipation of microwave-energy is conductedthrough the metal wall on which they are attached and this heat isthereafter can. In other respects the design mode of operation and Thewidth of the box is 1 inches wide. The length overall is 3 inches andthe height is it of an inch. The results obtained with this embodimentare comparable with that of the preferred embodiment althoughnumerically a larger number of elements are required and the overallvolume of the box is larger with this construction. The ferrite pieces40a, 40b, 40cand 40d located on the walls 53 and 54' are approximately5/16 inches in thickness while the dimensions of ferrite beads 39 arethe same as in the preceding example. Thus, althoughthe wall separationis actually greater than in the preceding example, and is 1 inches inwidth, the distance or spacing is the same. ThusLS" 2 (5/ I6) of aninch. Hence the calculation of effective impedance, Z, 377 (SIP) 33.51ohms.

The 0,, in this example is determined as in the preceding case: V,volume of 7 ferrite beads 0.740 cubic inches. V 3" X 1%" X 54" 3.37cubic inches. Thus Q V lV, 3.37/0.740 4.6. The measured value ofattenuation was 59 dbm.

The embodiment of FIG. 9 represents a special case I stantially the sameas the inner perimeter of the metal cavity. This embodiment includes thetwo spaced electrical conductors 61 and 63 which are insulated from oneanother and are adapted to be connected to a suitable supply of filamentcurrent and high voltage. The conductors are first surrounded or packedwith a lossy dielectric material such as Eccosorb CR8 or FDS,illustrated as element 65. In turn, the lossy dielectric is covered by ametal wrapping 67 which may be in the form of a metal tape. In addition,a metal container 69 is fitted over the ends of conductors 61 and 63andin electrical contact with the metal tape 67. Metal container 69 forms adome which covers the stem section of the magnetron and serves ashielding function. As illustrated the magnetron filter box is mountedon a section of the pole .piece 71 of the magnetron, the pole pieceserving as one of the electrically conductive walls of the filter box inthe cavity otherwise formed by the domelike member 69. In turn, thiswall effectively has a passage through which the magnetron stem portionis inserted and introduced within the filter box. FIG. 10a shows incross-section a portion of the filter box which includes the conductors61 and 63, the packing 65 of electrically lossy dielectric material, andthe underlying layer of metal tape 67. A modification of the foregoingis illustrated in FIG. 10b in which conductors 61' and 63' are firstensleeved by a dielectric material 73, and

this material is in turn covered by a heat shrinkable tube consisting oflossy material such as ferrite powder or both, within a large shieldingmetal enclosure, but no attempt was made to integrate the shieldingenclosure with the dissipative networks to form a compact and moreefficient magnetron filter box that comprised a fewer number of lowercost parts;

By the teachings of my invention the shielding is integrated into thesimple compact arrangement of the I drawings with the dissipativematerial so as to form structure analogous both to a low Q microwavecavity and a lossy transmission line, and in this more efficientlyeliminates the leakage of the fundamental frequency, leakage ofsubharmonics, and any fast modes which in the prior art structures werecapable of circulating between the filament lines and the large cavity.

It is understood that the foregoing embodiments of the invention arepresented as illustrative of the invention and not by way of limitation,since one skilled in the art, upon reviewing this specification, findsmany equivalents and substitutions for the details of the elementsillustrated and which substitutions or equivalents do not depart fromthe spirit and scope of the disclosed invention.

Accordingly, it is specifically understood that my invention is to bebroadly construed within the spirit and scope of the appended claims.

What I claim is:

1. A combination of a magnetron and magnetron filter box comprising:

a magnetron which includes a body portion, said body portion having anelongated stemlike end, and said stemlike end containing cathode andfilament terminals, a pole piece of ferrous material, said pole piececontaining an opening therethrough, said body portion positioned withsaid stemlike end extending through said opening and projecting outwardfrom the surface of said pole piece, and means preventing microwaveenergy leakage through said pole piece opening between said body portionand said pole piece;

first and second elongated electrical conductor means for carryingelectrical filament current and a high cathode electrical voltage, saidelectrical conductor means being insulated electrically from one anotherand having first respective end portions adjacent one another and eachconnected to a respective one of the cathode and filament terminals ofsaid microwave tube;

microwave dissipative material for dissipating microwave energy andcomprising a predetermined volume, V,, said microwave dissipativematerial surrounding said first and second conductor means and extendingtherealong a major portion of the length thereof each to a pointproximate said end portions of said conductor means;

microwave cavity means defining a predetermined region of volume V, forconfining microwave energy therewithin, said cavity comprising walls ofelectrically conductive material and including as one wall thereof aportion of said pole piece surface, said microwave cavity includingtherein said electrical conductor means, said stemlike ends of saidmagnetron body and said microwave dissipative material;

, 15 means for providing a filament current and high voltage path tosaid conductor means from outside said cavity; and wherein the volume ofsaid cavity, V and the volume of said dissipative material, V,, define aratio of V to V,, which ratio comprises a quantity equal to or less than20 whereby said microwave cavityis of a low Q factor electricalcharacteristic;

and i 1 wherein a predetermined impedance Z is defined as approximatelyequal to 377 (S P) ohms, wherein said relationship of S and P being suchthat said Z is equal to or less than 50 ohms, where S is defined as anaverage spacing distance between the outer surface of said dissipativematerial and the inner wall surfaces of said cavity means and where P isdefined as amean perimeter length about the space between saiddissipative material and said cavity walls,

I 2. The invention as defined in claim 1 further com-' prising a slab ofmicrowave dissipative material located on a wall of said microwavecavity at a location adjacent said st'emlike end of said magnetron bodyportion.

3. The invention as defined in claim 2 further comprising a plurality ofsmall holes less than 1/ th A in diameter located, in at least somewalls of said microwave cavity to provide a path out of and into saidcavity for air.

4. An; electromagnetic energy filter means comprismg: g

first and second elongated electrical conductor means forcarryingelectrical filament current and a high cathode electrical voltage, saidelectrical conductor means being spaced and having end portions adjacentone another for connection to the respective terminals of a microwavetube; microwave dissipative 7 material for dissipating microwave energy,said microwave dissipative material surrounding said first and secondconduc-- tor means and extending along a predetermined length thereofupto 'a point proximate said end portions of said conductor means, saiddissipative 7 material further comprising a predetermined volume V,;

microwave cavity means for confining microwave energy within apredetermined region of volume V',, said cavity comprising walls ofelectrically conductive material, said cavity means includingtherewithin said microwave dissipative material, and said first andsecond conductor means, including said end portion of each saidconductor means;

and wherein the ratio of cavity. volume, V,,, to dissipative materialvolume,,V,, comprises a quantity equal to or less than 100 to define alow Q cavity;

a first opening in a first wall of said cavity means, said first openingbeing-of sufficient size to permit positioning within saidcavity meansof a terminal socket end of a microwave tube for permitting connectionwithin said cavity means between terminals on said socket and thecorresponding end portions of said conductor means and to permit I saidmicrowave tube to close said first opening;

means for permitting application of filament voltage and high'voltage tosaid conductor means from outside'said cavity;

I and wherein an average spacing'distance is defined between the outersurface of said dissipative material and an inner surface of said cavitymeans which comprises a predetermined lengthS, and wherein a meanperimeter length, P, is defined about the space between said.dissipative material and said cavity walls, and wherein a predeterminedimpedance, Z,,, is defined as approximately equal to the quantity 3778/? ohms, said relationship of S and P being such that said Z, is equalto or less than ohms.

5. The invention as defined in claim 4 wherein said electrical conductormeans are'sufficiently spaced from said cavity walls to provide highvoltage insulation therebetween.

6.- The invention as defined in claim 5 whereinsaid ratio of V /V, isequal to or less than 20.

7. The invention as defined inclaim 4 wherein said cavity meanscomprises the metal box open along one side, and a metal pole piece of amicrowave tube closes said open side to complete said metal cavity, andwherein said tube socket extends through said pole piece into saidcavity,

- 8. The invention as defined in claim 7 further comprising a pluralityof additional openings in said metal box for permitting passage of air,each of said openings having as its longest dimension-a length equal toor less.

than the quantity 1/10.), where A is the wavelength of the operatingfrequency of the magnetron.

9. The invention as defined in claim 8 further comprising an additionalquantity of microwave dissipative material located along an end wall ofsaid cavity adjacent said first opening.

10. The invention as defined in claim 8 wherein said dissipativematerial further comprises a plurality of discrete portions arranged inside by side relationship,

each of' said portions having a passage therethrough and wherein saidconductor extends through said passages for supporting said portions,and a single sheath of heat shrinkable polyvinyl tubing materialextending over and covering all said portions to retain saiddiscreet-portions in said side by side relationship.

' l1.-T he combination of a magnetron and magnetron filter boxcomprising:

a magnetron microwave tube for generating microwave frequency energy ofa wavelength A; said magnetron microwave tube including a tube body,microwave passage means for permitting passage of generated microwaveenergy from said tube bodyto a load, and filament and high voltageterminal means'for coupling filament current and high voltage to withinsaid tube body; low Q microwave cavity of a predetermined volume, V,,covering saidterminal means, said low Q microwave cavity includingtherewithin elongated electrical conductor means adapted for connectionto an external source of filament current and high voltage for providingan electrically conductive path to said'terminals, and microwavedissipative material of a predetermined volume, V surrounding saidconductor means along a substantial portion of the length of saidconductor means for dissipating microwave energy exiting from saidmagnetron tube via said terminals, and wherein the outer surface ofsaid-dissipative material'and the inner surface of said microwave cavityis separated by an average spacing distance of a predetermined length,S, and wherein a mean perimeter length, P, is defined about said spacebetween said dissipative material and the surrounding walls of saidcavity; and wherein a predetermined impedance, Z,,, is defined asapproximately equal to the quantity 377 (SP) ohms,

, said relationship of S and P being such that Z. is

equal to or less than 50 ohms, and wherein the ratio V /V, is a quantityequal to or less than 100.

12. The invention as defined in claim 11 wherein said dissipativematerial further comprises a'plurality of discreet portions arranged inside by side relationship,

each of said portions having a passage therethrough and wherein saidconductor extends through said passages for supporting said portions,and a single sheath of heat shrinkable polyvinyl tubing materialextending over and covering all said portions to retain said discreetportions in said side by side relationship.

13. The invention as defined in claim 11 wherein said microwave cavitymeans includes a plurality of openings through the walls thereof topermit passage of cooling air into said cavity, each of said passageshaving as its longest dimension a length less than 1/1 0th A.

14. The invention as defined in claim 1 1 wherein said ratio of V to V,is approximately 20.

1. A combination of a magnetron and magnetron filter box comprising: amagnetron which includes a body portion, said body portion having anelongated stemlike end, and said stemlike end containing cathode andfilament terminals, a pole piece of ferrous material, said pole piececontaining an opening therethrough, said body portion positioned withsaid stemlike end extending through said opening and projecting outwardfrom the surface of said pole piece, and means preventing microwaveenergy leakage through said pole piece opening between said body portionand said pole piece; first and second elongated electrical conductormeans for carrying electrical filament current and a high cathodeelectrical voltage, said electrical conductor means being insulatedelectrically from one another and having first respective end portionsadjacent one another and each connected to a respective one of thecathode and filament terminals of said microwave tube; microwavedissipative material for dissipating microwave energy and comprising apredetermined volume, Vl, said microwave dissipative materialsurrounding said first and second conductor means and extendingtherealong a major portion of the length thereof each to a pointproximate said end portions of said conductor means; microwave cavitymeans defining a predetermined region of volume Vc for confiningmicrowave energy therewithin, said cavity comprising walls ofelectrically conductive material and including as one wall thereof aportion of said pole piece surface, said microwave cavity includingtherein said electrical conductor means, said stemlike ends of saidmagnetron body and said microwave dissipative material; means forproviding a filament current and high voltage path to said conductormeans from outside said cavity; and wherein the volume of said cavity,Vc, and the volume of said dissipative material, Vl, define a ratio ofVc to Vl, which ratio comprises a quantity equal to or less than 20whereby said microwave cavity is of a low Q factor electricalcharacteristic; and wherein a predetermined impedance Zo is defined asapproximately equal to 377 (S divided by P) ohms, wherein saidrelationship of S and P being such that said Zo is equal to or less than50 ohms, where S is defined as an average spacing distance between theouter surface of said dissipative material and the inner wall surfacesof said cavity means and where P is defined as a mean perimeter lengthabout the space between said dissipative material and said cavity walls.2. The invention as defined in claim 1 further comprising a slab ofmicrowave dissipative material located on a wall of said microwavecavity at a location adjacent said stemlike end of said magnetron bodyportion.
 3. The invention as defined in claim 2 further comprising aplurality of small holes less than 1/10th lambda in diameter located inat least some walls of said microwave cavity to provide a path out ofand into said cavity for air.
 4. An electromagnetic energy filter meanscomprising: first and second elongated electrical conductor means forcarrying electrical filament current and a high cathode electricalvoltage, said electrical conductor means being spaced and having endportions adjacent one another for connection to the respective terminalsof a microwave tube; microwave dissipative material for dissipatingmicrowave energy, said microwave dissipative material surrounding saidfirst and second conductor means and extending along a predeterminedlength thereof up to a point proximate said end portions of saidconductor means, said dissipative material further comprising apredetermined volume Vl; microwave cavity means for confining microwaveenergy within a predetermined region of volume Vc, said cavitycomprising walls of electrically conductive material, said cavity meansincluding therewithin said microwave dissipative material, and saidfirst and second conductor means, including said end portion of eachsaid conductor means; and wherein the ratio of cavity volume, Vc, todissipative material volume, Vl, comprises a quantity equal to or lessthan 100 to define a low Q cavity; a first opening in a first wall ofsaid cavity means, said first opening being of sufficient size to permitpositioning within said cavity means of a terminal socket end of amicrowave tube for permitting connection within said cavity meansbetween terminals on said socket and the corresponding end portions ofsaid conductor means and to permit said microwave tube to close saidfirst opening; means for permitting application of filament voltage andhigh voltage to said conductor means from outside said cavity; andwherein an average spacing distance is defined between the outer surfaceof said dissipative material and an inner surface of said cavity meanswhich comprises a predetermined length S, and wherein a mean perimeterlength, P, is defined about the space between said dissipative materialand said cavity walls, and wherein a predetermined impedance, Zo, isdefined as approximately equal to the quantity 377 S/P ohms, saidrelationship of S and P being such that said Zo is equal to or less than50 ohms.
 5. The invention as defined in claim 4 wherein said electricalconductor means are sufficiently spaced from said cavity walls toprovide high voltage insulation therebetween.
 6. The invention asdefined in claim 5 wherein said ratio of Vc/Vl is equal to or less than20.
 7. The invention as defined in claim 4 wherein said cavity meanscomprises the metal box open along one side, and a metal pole piece of amicrowave tube closes said open side to complete said metal cavity, andwherein said tube socket extends through said pole piece into saidcavity.
 8. The invention as defined in claim 7 further comprising aplurality of additional openings in said metal box for permittingpassage of air, each of said openings having as its longest dimension alength equal to or less than the quantity 1/10 lambda where lambda isthe wavelength of the operating frequency of the magnetron.
 9. Theinvention as defined in claim 8 further comprising an additionalquantity of microwave dissipative material located along an end wall ofsaid cavity adjacent said first opening.
 10. The invention as defined inclaim 8 wherein said dissipative material further comprises a pluralityof discrete portions arranged in side by side relationship, each of saidportions having a passage therethrough and wherein said conductorextends through said passages for supporting said portions, and a singlesheath of heat shrinkable polyvinyl tubing material extending over andcovering all said portions to retain said discreet portions in said sideby side relationship.
 11. The combination of a magnetron and magnetronfilter box comprising: a magnetron microwave tube for generatingmicrowave frequency energy of a wavelength lambda ; said magnetronmicrowave tube including a tube body, microwave passage means forpermitting passage of generated microwave energy from said tube body toa load, and filament and high voltage terminal means for couplingfilament current and high voltage to within said tube body; a low Qmicrowave cavity of a predetermined Volume, Vc, covering said terminalmeans, said low Q microwave cavity including therewithin elongatedelectrical conductor means adapted for connection to an external sourceof filament current and high voltage for providing an electricallyconductive path to said terminals, and microwave dissipative material ofa predetermined volume, Vl, surrounding said conductor means along asubstantial portion of the length of said conductor means fordissipating microwave energy exiting from said magnetron tube via saidterminals, and wherein the outer surface of said dissipative materialand the inner surface of said microwave cavity is separated by anaverage spacing distance of a predetermined length, S, and wherein amean perimeter length, P, is defined about said space between saiddissipative material and the surrounding walls of said cavity; andwherein a predetermined impedance, Zo, is defined as approximately equalto the quantity 377 (S Divided by P) ohms, said relationship of S and Pbeing such that Zo is equal to or less than 50 ohms, and wherein theratio Vc/Vl is a quantity equal to or less than
 100. 12. The inventionas defined in claim 11 wherein said dissipative material furthercomprises a plurality of discreet portions arranged in side by siderelationship, each of said portions having a passage therethrough andwherein said conductor extends through said passages for supporting saidportions, and a single sheath of heat shrinkable polyvinyl tubingmaterial extending over and covering all said portions to retain saiddiscreet portions in said side by side relationship.
 13. The inventionas defined in claim 11 wherein said microwave cavity means includes aplurality of openings through the walls thereof to permit passage ofcooling air into said cavity, each of said passages having as itslongest dimension a length less than 1/10th lambda .
 14. The inventionas defined in claim 11 wherein said ratio of Vc to Vl is approximately20.